Regularly, I thank them for the data and usually invite them to join us, and if they have given me a credible lead I will chase it down and add it to the relevant profiles.
A lot of them are actually not in Wikitree due to it being mainly in English, I know I haven't been doing anything on the French portal project lately, but it is something that needs to happen. Unfortunately, from what Chris said previously, we can't get a fully multilingual program as exists elsewhere due to various considerations, financial being one of them.